This invention relates to a laser welding process that attaches a metal sheet to a ceramic substrate. More particularly, this invention relates to a process that includes welding a metal sheet to a gold-plated tungsten pad preformed on the substrate.
In the design of microelectronic components, it is common to include a sheet metal element affixed to a ceramic substrate. For example, in an oscillator crystal assembly, a crystal is mounted upon sheet metal tabs, which tabs are in turn mounted upon a ceramic substrate. In common practice, metal pads are preformed on the substrate, and the tabs are bonded to the pads using an epoxy adhesive. The bond pads are formed of tungsten that is plated with a nickel thin film and an outermost gold thin film that provides a suitable surface for bonding to the adhesive. Problems arise in reliably dispensing the adhesive in the precise microquantities required to produce an adequate bond without seeping onto adjacent areas. Also, there is a tendency for the tab to shift position prior to curing. Still further, although the adhesive contains silver particles to enhance electrical conductivity, the bond nevertheless exhibits increased electrical resistance that interferes with electrical current that is applied to the crystal through the tabs via the pads.
Laser welding of metals is generally known. Since ceramic does not weld to metal, it has been proposed to apply a metal pad to the ceramic substrate to provide a base for welding. However, thermal cycling that accompanies fusion of the metal pad tends to produce microcracking in the underlying ceramic or spalling of the metal pad from the ceramic. Thus, a process is desired for laser welding a metal sheet to a metal pad on a ceramic substrate which avoids fusion of the pad.